2-28 Flagella are one type of structure used for motility

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• Flagella are semi-rigid structures used to move microbial cells.
• Flagella are mostly composed of flagellin, a protein polymer of repeated subunits. Flagella are attached to the cell through a complex of proteins called the hook and basal body.
• Flagella cause the cell to move by their rotation, which is powered by the proton motive force.

Surface structures are typically attached to a membrane and extend into the environment. Important structures include flagella [7], pili [8], fimbriae [9], and glycocalyx [10]. These protrusions and surfaces interact with the environment around the microorganism and therefore are pivotal in how the microbe sees the world and how we see the microbe.

In many bacteria, flagella are responsible for motility in liquid. There is also a loose correlation between cell shape and the presence of flagella. Almost all spirilla, half of all rod-shaped bacteria, and only a few of the cocci are motile by flagella. In fact, most cocci are non-motile. One rationale for this correlation might be that spherical cells such as the cocci simply do not have the best geometry for directional movement by flagella, unlike more linear bacteria.

Flagella can be thought of as little semi-rigid propellers that are free at one end and attached to a cell at the other. Flagella are thin (20 µm) and long with some having a length 2-3 times (about 10 µm) the length of the cell. Due to their small diameter, flagella cannot be seen in the light microscope unless a special stain is applied. Bacteria can have one or more flagella arranged in clumps or spread over the cell surface. Figure 2-46 demonstrates some of the more common arrangements.

Chemical Structure

Flagella are mostly composed of the protein flagellin, which is bound in long chains and wraps around itself in a left-handed helix as shown in Figure 2-47 [11]. The number of protein monomers that it takes to make a single turn of the helix is determined by the protein subunits themselves.

The flagellum is attached to the cell through complex protein structures termed the hook and the basal body. One ring in the basal body rotates relative to the other causing the flagellum to rotate. The energy to drive the basal body is obtained from the proton motive force. In some fashion the translocation of protons from outside to inside the membrane causes the rotation of the flagellum. In a sense, the protons move through the wheel-like structure of the basal body (similar to a water wheel, except using protons) and this causes the rotation of the assembly including the flagellum. When E. coli is swimming through a solution the flagella turn counter-clockwise and push the microbe through solution. This behavior is termed smooth swimming. It is possible for E. coli to also reverse the direction of flagellar rotation and when the flagella turn clockwise, they pull against the bacterial cell. Since E. coli is flagellated peritrichously (that is, at many positions), it is pulled in all directions and tumbles.

How fast do bacterial cells move? They average 50 µm/sec, which is about 0.00015 kilometers/hr. This may seems slow but remember their tiny size. Figure 2-66 [12] shows a better comparison and indicates that relatively speaking, bacteria are faster than humans. Also remember that this motility happens in water, which is much more viscous than air.

If a flagellum breaks off it is resynthesized until it reaches the appropriate length. This growth actually occurs from the tip. The flagellar filament is hollow and flagellin monomers are passed up through this space until they reach the growing tip and are added to the structure.

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